Before being fed into a power plant combustion chamber, coal is pulverized into dust-like "fines" by a pulverizer, and then forced by a powerful exhauster fan from the pulverizer through a branched arrangement of ducts or chutes which feed burner nozzles in the combustion chamber. The exhauster fan first throws the coal radially into a primary discharge chute. The coal/air flow leaving the exhauster fan is uneven, in that the heaviest particles tend to be radially centrifuged out toward the back or bottom of the exhauster outlet, and against one side or wall of the chute. Accordingly, the coal/air flow leaving the fan and proceeding down the chute to the burners tends to be light on one side and heavy on the other side in terms of both particle size and distribution.
The prior art solution has been to provide "riffle boxes" in the chute between the fan and the burners. A riffle box is a series of vertical, spaced plates separated by angled separator bars with alternating orientation from plate to plate. The theory is that the separator bars on one plate will deflect the coal in one direction, while the separator bars on adjacent plates will deflect the coal in the opposite direction, thereby splitting and redistributing the flow for a more homogeneous mixture. The typical arrangement is to provide a series of riffle boxes, with a first riffle box splitting the flow like a "Y" into two chute branches, and a subsequent riffle box on each of the first two branches splitting the flow again into a total of four chutes. Each chute typically fuels one of four corner-mounted burners in a tangentially-fired combustion chamber (best shown in FIG. 2).
The riffle boxes have proven ineffective in providing a more homogeneous mixture to the burners, and the coal/air flow reaching the four combustion chamber burners differs significantly from burner to burner. Several problems result: too lean a mixture at a burner can create NOX; oversized particles and inefficient burning create LOI (loss on ignition) contamination of the ash byproduct and reduced combustion efficiency; and, perhaps most importantly, the out-of-balance burner flow distorts the combustion chamber fireball from the ideal spherical shape to an undesirable elliptical shape, creating hot and cold spots in the boiler tubes and causing gas control problems.